Bones and Skeletal Tissues

Overview

The skeletal system is composed of bones and cartilages that provide the framework for the human body. Bones are dynamic organs with multiple functions essential for movement, protection, and homeostasis.

Functions of Bones

Major Functions

• Support: Bones provide structural support for the entire body and soft organs.

• Protection: Bones protect vital organs such as the brain (skull), spinal cord (vertebral column), and thoracic organs (rib cage).

• Movement: Bones act as levers for muscle action, enabling movement.

• Mineral and Growth Factor Storage: Bones serve as reservoirs for minerals, especially calcium and phosphorus, and store growth factors.

• Blood Cell Formation: Hematopoiesis occurs in red marrow cavities, producing blood cells.

• Triglyceride (Fat) Storage: Fat is stored in bone cavities and used as an energy source.

• Hormone Production: Bones secrete osteocalcin, a hormone that regulates insulin secretion, glucose levels, and metabolism.

Organization of the Human Skeleton

Axial and Appendicular Skeleton

• Axial Skeleton: Forms the long axis of the body and includes the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Comprises the bones of the upper and lower limbs and the girdles (shoulder and pelvic) that attach limbs to the axial skeleton.

Example: The femur (thigh bone) is part of the appendicular skeleton, while the sternum is part of the axial skeleton.

Classification of Bones by Shape

Types of Bones

• Long Bones: Longer than they are wide (e.g., humerus, femur).

• Short Bones: Cube-shaped (e.g., bones of the wrist and ankle).

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Bone Structure

Bone as an Organ

• Bones are organs composed of several tissue types: osseous (bone) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels.

Levels of Structure

• Gross Anatomy: Macroscopic structure visible to the naked eye.

• Microscopic Anatomy: Structure visible under a microscope.

• Chemical Composition: Molecular and elemental makeup of bone tissue.

Types of Bone Tissue

Compact and Spongy Bone

• Compact Bone: Dense outer layer that appears smooth and solid; provides strength and support.

• Spongy Bone: Internal layer with a honeycomb structure (trabeculae); spaces are filled with red or yellow bone marrow.

Structure of Bones by Type

Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone (diploë) covered by compact bone.

• Compact bone is sandwiched between connective tissue membranes: periosteum (outer) and endosteum (inner).

• Bone marrow is scattered throughout spongy bone; no defined marrow cavity.

• Hyaline cartilage covers areas involved in movable joints.

Long Bones

• Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding a medullary cavity (yellow marrow in adults).

• Epiphyses: Bone ends; external compact bone, internal spongy bone; articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of the epiphyseal plate (site of bone growth in childhood).

Bone Membranes

Periosteum

• White, double-layered membrane covering external bone surfaces (except joints).

• Fibrous Layer: Outer layer of dense irregular connective tissue; contains Sharpey's fibers anchoring it to bone.

• Osteogenic Layer: Inner layer containing osteogenic stem cells that give rise to most bone cells.

• Rich in nerve fibers and blood vessels; provides anchoring points for tendons and ligaments.

Endosteum

• Delicate connective tissue membrane covering internal bone surfaces, including trabeculae and canals.

• Contains osteogenic cells for bone growth and repair.

Bone Marrow and Hematopoietic Tissue

Red and Yellow Marrow

• Red Marrow: Found in trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum).

• In newborns, all spongy bone and medullary cavities contain red marrow.

• In adults, red marrow is mainly in the heads of femur and humerus, flat bones, and some irregular bones (e.g., hip bone).

• Yellow Marrow: Stores fat; can convert to red marrow if the body becomes anemic.

Bone Markings

Types and Functions

• Projections: Outward bulges for muscle, ligament, and tendon attachment; may also form joints.

• Depressions: Bowl- or groove-like cutouts serving as passageways for vessels and nerves or playing a role in joints.

• Openings: Holes or canals for passage of blood vessels and nerves.

Microscopic Anatomy of Bone

Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), rich in collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress/strain sensors.

• Bone-Lining Cells: Flat cells on bone surfaces; help maintain matrix. Called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Giant, multinucleate cells derived from hematopoietic stem cells; responsible for bone resorption (breakdown).

Compact Bone (Lamellar Bone)

• Osteon (Haversian System): Structural unit; elongated cylinder running parallel to bone's long axis, composed of concentric rings (lamellae).

• Lamellae: Collagen fibers in different directions in adjacent rings; withstand stress and resist twisting.

• Central (Haversian) Canal: Runs through osteon core; contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Perpendicular to central canal; connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Tiny canals connecting lacunae; allow communication and nutrient/waste exchange between osteocytes.

• Interstitial Lamellae: Fill gaps between osteons or are remnants of old osteons.

• Circumferential Lamellae: Extend around the entire diaphysis; help resist twisting.

Spongy Bone

• Composed of trabeculae aligned along lines of stress for strength.

• No osteons; trabeculae contain irregularly arranged lamellae and osteocytes connected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes bone cells (osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts) and osteoid (unmineralized bone matrix).

• Osteoid consists of ground substance and collagen fibers, providing tensile strength and flexibility.

• Resilience is due to sacrificial bonds in or between collagen molecules that dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (Mineral Salts): Make up 65% of bone by mass; mainly tiny calcium phosphate crystals in and around collagen fibers.

• Responsible for bone's hardness and resistance to compression.

• Bones are as strong as steel in resisting tension and half as strong as steel in resisting compression.

Example: The mineral composition of bones allows them to last long after death and provide information about ancient populations.

Sample Questions (from Slides)

• Which of the following is NOT a function of bones? (Answer: Transportation)

• The principal component of bone that contributes to hardness is: (Answer: Hydroxyapatite)